For the treatment of exhaust gas from cars and the like, there has been known a method in which catalysts are carried in honeycomb structures made of metal or ceramics.
Various types of honeycomb structures have been proposed. For example, one of the known honeycomb structures has a large number of inner cells inside thereof which extend longitudinally along the length of the honeycomb structure and have open ends on both sides thereof. Another type of the known honeycomb structure also has a large number of longitudinally arranged cells inside thereof which have an open end on one side and a closed end on the other side thereof and, are arranged alternately so that every other adjacent cell has an open end (or a closed end) on one side and a closed end (or an open end) on the other side of the honeycomb structure and communicate with each other through holes in the walls of the cells.
In order to have catalyst components carried on the walls of the cells of the honeycomb structure of the above-described type in a suitable manner, it is necessary to coat various liquids including, among others, a slurry which contains catalyst components on the walls of the cells.
Examples of the liquids to be applied to the walls of the cells are as follows.
(a) Chemicals to be used in pretreatment of post-treatment in the production of catalysts, for example, acids, aqueous alkali solutions, and organic substances. These are aqueous solutions which contain no catalyst component.
(b) Aqueous solutions containing catalyst components such as aqueous solutions of palladium compounds, aqueous solutions of platinum compounds, and aqueous solutions of rhodium compounds.
(c) Slurries containing catalyst components and mainly composed of catalyst components such as platinum, palladium and rhodium, refractory metal oxides such as aluminum oxide and cerium oxide. The slurries may contain one or more of various metal compounds, acids, organic substances and the like. Generally, the slurries have a high viscosity (e.g., on the order of from 100 to 500 cps), a specific gravity of from 1.0 to 2.0 g/ml, and a component particle size of no greater than 10 .mu.m.
(d) Slurries mainly composed of refractory metal oxides such as aluminum oxide and cerium oxide. The slurries may contain one or more of various metal compounds, acids, organic substances and the like. They contain no catalyst component represented by precious metals. Their physical characteristics are the same as the slurries described in (c) above.
As a coating method for applying the above-described various liquids to the walls of numerous cells in the honeycomb structure having the construction described above, a method has been used in which a honeycomb structure in a vertical position is dipped in a desired liquid contained in a storage tank, the honeycomb structure is then taken out from the liquid bath and left to stand as is for a while to extract or separate the liquid from the honeycomb structure by free fall of the liquid, and the liquid still remaining in the cells is removed forcibly by air-knife which blasts high-pressure air.
However, the conventional method has the following problems.
Firstly, it takes a considerably long time for the liquid to adhere all over the walls of the inner cells in the honeycomb structure, which makes it difficult to carry out the treatment efficiently. The cells have a considerably small sectional area and the pressure posed on the liquid while it rises within the cell is generated by the difference between the level of the liquid surface in the storage tank and that of the liquid surface in the cell. Hence, this pressure is not so high. In addition, usually the liquid to be coated has a high viscosity. Therefore, in the above-described method, the speed at which the liquid rises within the cells is not high enough.
Secondly, the amount of the liquid which adheres to the wall of the cells is not equal from cell to cell. Upon the separation of the liquid, the liquid is left to fall free followed by the removal of the liquid which is carried out by using an air-knife. As a result, it is often the case that a large amount of the liquid remains in some cells while only a small amount of the liquid remains in other cells, thus failing to cause the liquid to adhere to the honeycomb structure uniformly. It is desirable and important to uniformly distribute catalyst components and the like on the walls of the inner cells in the honeycomb structure because in the case of the treatment of exhaust gas, for example, the reaction occurs when the exhaust gas contacts the catalyst component on the honeycomb structure.
Thirdly, unnecessary liquid adheres to the side surface of the honeycomb structure. The right portion where liquid is to adhere is the walls of the inner cells in the honeycomb structure through which the exhaust gas passes, but it is unnecessary to allow the liquid to adhere on the side surface of the honeycomb structure. The liquid adhering on the side surface is of no use.
As described hereinbelow, in the liquid coating method and apparatus for the honeycomb structure of the present invention, a honeycomb structure, which is an article to be held, having a shape of a circular column, an elliptic column, a circular cylinder, an elliptic cylinder, or the like, must be held and the outer periphery of the article to be held must be covered partly or entirely.
For this purpose, a holder can be used which is composed of two halves, each having a shape of a longitudinally split circular cylinder provided with a recess or groove that defines an inner cavity having a contour corresponding to that of the article to be held when the halves are put together in close contact with each other.
However, if the size and shape of the article to be held, i.e., honeycomb structure, fluctuate, a problem arises that when the article to be held is relatively large, the holder is closed incompletely to fail to establish a complete seal, and on the other hand, when the article to be held is relatively small, it does not contact the holder closely, resulting in that the force of holding given by the holder is not strong enough.
A method and an apparatus might be considered useful in which articles to be held are held by using a rubber tube having an inner diameter sufficiently larger than the outer diameters of the articles to be held, and air is forcibly introduced into the tube to reduce its inner diameter to hold the articles in close contact therewith.
However, in the above-described apparatus, a problem arises that when air is introduced into the rubber tube, wrinkles tend to occur in the rubber from part to part, resulting in that the sealing of those parts with wrinkles is incomplete.
For example, when it is contemplated to coat a liquid on an inside surface of an article to be held having a shape of a circular cylinder and provided only inside thereof with one or more tubes extending in an axial direction, as honeycomb structure, the article to be held is held by a holder, a liquid tank is coupled with the lower end of the article, and a liquid is supplied from the liquid tank and forced to rise into the inside of the article to coat the liquid only on the inside thereof. In this case, the use of the conventional holder with a rubber tube results in the occurrence of wrinkles in the rubber as earlier explained, and the liquid forced upward flows upward between the article and the rubber tube via channels formed by the wrinkles and thus the liquid adheres also to the outside of the article, thus failing to achieve the purpose.
The conventional technique suffers from a problem that when the size and shape of the article to be held fluctuate more or less, it is difficult to hold the article certainly, and in addition the seal between the article and the holder is insufficient.